If your repayment strategy is based on fiat, then would you consider giving your depositors the option to convert their debt into fiat (preferably EUR or USD)? I mentioned it in hashking's thread but apparently no one likes that solution. According to that scheme, fiat based payments are faster (with regards to total fiat debt) if the value of BTC exchange rate increases, and slower if it decreases, so there is an incentive to convert for the lenders and borrower if they predict an increased exchange rate. If your prediction is undecided, best thing would be to convert half of the debt.